1. Field Of The Invention
The present invention relates to tamper evident packages and more particularly to tamper evident packages having container lids constructed from thin, tearable materials which are readily applied to containers filled during packaging operations yet which are not removable from the containers without tearing and giving evidence of the container having been opened.
2. Prior Art
Tamper evident containers for food or perishable materials being sold at retail have been in regular usage. In most cases these types of containers were constructed for use with removable lids which positively sealed and locked the container closed during handling and shipment and while on sale at retail. These lids also permitted the container to be easily opened by the purchaser and sealingly reclosed to store partial contents.
The major consumers of tamper evident food containers and lids are food producers, such as commercial dairies, which in turn supply the grocery trade with prepackaged goods. These kinds of packages utilize packaging materials on a mass basis in high speed material handling equipment. Criteria by which these kinds of packagers select containers to be used for their goods include cost, ease of use in the existing packaging operations, performance of the package during product shipment, degree of evidence of tampering provided, ability of the purchaser to open and reclose the package after purchase, and appearance of the package.
Many prior art containers and lids have been complex, relatively expensive constructions employing lid locking structures and tear strips permitting removal of the lid locking structures only in a way which made such a removal evident to those viewing the container. These lids and containers were frequently formed from injection molded parts, or the equivalent, which had complicated molded-in locks and tear strips formed by lines of weakness molded into the lids.
These injection molded parts were expensive to use by packagers of food and perishable products. The containers and lids generally were designed to specially conform to each other. The capital expenses required by the container manufacturer for creating production tooling was relatively high. Moreover, container production speeds were limited because of the relatively slow molding processes. The total end-product price, using such packaging, thus tended to be relatively high.
Moreover, some packaging designs did not function well during shipment. In some designs the lids were equipped with projecting pull tabs or with flange-like shoulders. These were sometimes forced into engagement with adjacent packages when packed in shipping crates. Vibration or jostling during shipment in these circumstances sometimes caused partial removal or loosening of the lids. In other cases the lids were not removed or loosened but the tear strips were broken thus falsely indicating that the container had been opened. In either case the product was not salable.
Other designs were found to be difficult for purchasers to use because the initial removal of the lid from the container was physically difficult, or was too complicated to be adequately explained in printing on the lid itself, or because the lids did not adequately evidence the fact that the associated container had been opened.
Alternative proposals for package constructions involved manufacturing the containers and lids from "thermo-formed" plastics. These parts were produced by forming sheet plastic on molds while applying heat and pressure. Parts so constructed can be rapidly produced in great volumes and thus tend to be quite inexpensive. The nature of the thermo-forming process is such that the containers and lids produced by it have relatively uniform thin walls. Thus these parts were not readily formed into lid structures having tear strips with thick, rigid wedge-like teeth, or projecting locking fingers for locking against a container lip. Thermo-formed parts were better adapted to be used for sealing products in a way where "tampering" (or lid loosening) was not evidenced by the lid itself. As a consequence thermo-formed parts (or other parts constructed from thin sheets of material) were not widely used as tamper evident containers, with a few exceptions.
One proposal for using thermo-formed lids in a tamper evident application was to form a more or less standard lid, seal it onto a container of food or other product, and then apply a band of shrink-wrap plastic around the lid so that the lid could not be removed from the container without first tearing and removing the shrink wrap. In a variation on this theme, it has been proposed that the lid itself be formed from a shrinkable material which, after the lid is installed on the container, is shrunk about the container in such a way that the lid can not be removed from the container without first tearing and removing a tear strip portion of the lid.
These approaches had the disadvantage of requiring the packager to perform additional, sometimes expensive and time consuming, steps in the packaging process. For example, shrink bands had to be assembled to the packages after the lid was in place and the shrinking process itself was often conducted by heating the shrink bands. Thus, in addition to extra handling, the heating requirement resulted in a need for heating equipment in the packaging line and sometimes resulted in heating the packaged product itself which was usually undesirable.